1. Field of the Invention
This invention relates to methods of providing printing form precursors. The invention relates further to such precursors per se, and to their use. The invention also relates to the reclaiming of printing form precursors which do not meet an acceptable specification, to bring such precursors within specification.
This invention relates primarily to positive working printing form precursors. In such precursors the coating after imagewise exposure becomes more soluble in the exposed areas than in the non-exposed areas, in a chemical developer. The exposed areas are selectively dissolved away and only the remaining areas of the coating are ink-receptive.
2. Background Information
For many years the coatings used for positive working printing form precursors comprised alkali soluble resins and naphthoquinone diazide (NQD) derivatives, either as functional groups on the resins or as separate compounds. These coatings are imaged using flood UV radiation, delivered via a mask. These coatings have good stability over time.
In recent years there has been a move towards printing form precursors which can be imaged using IR lasers which have digital output and are computer-controlled, allowing the imaging step to be controlled by an operator from a computer screen.
The use of a digital process employing IR lasers has important advantages over the earlier UV methods but there is the disadvantage that the properties of the IR-sensitive printing form precursors tend not to be as stable over time, as those of the UV-sensitive precursors.
Thus, it has been observed that with many IR-sensitive positive working printing form precursors there may be a reduction in xe2x80x9csensitivityxe2x80x9d over time, after the coating has been applied to a substrate and dried, this effect being the result of reduced developer solubility of the unexposed coating with time prior to exposure. In this specification xe2x80x9csensitivityxe2x80x9d is referred to in the context of the entire process of exposure and development. It does not refer only to the matter of how the areas of the coating which are exposed react to that exposure. In the lithographic printing art this xe2x80x9csensitivityxe2x80x9d is sometimes called xe2x80x9coperating speedxe2x80x9d, or the skilled person may say that the coating has become xe2x80x9cslowerxe2x80x9d.
It is difficult for an operator to adjust for precursors whose sensitivity has reduced substantially (i.e. outside a defined specification). Therefore it would be desirable to have a method which improves precursors having positive working heat or IR-sensitive coatings, such that an operator has a more consistent and stable product.
Lithographic printing form precursors which can be imaged using IR lasers are described in WO 97/39894. Fundamentally the change in solubility on imaging using IR lasers are caused by heat not by chemical breakdown (xe2x80x9cphotolysisxe2x80x9d) in the coating. The heat is produced by the interaction of the IR radiation and IR absorbers present in the coatings, and acting as light-to-heat converters.
In order to provide IR and/or heat sensitive precursors with more even properties over time a stabilizing heat treatment was described in WO 99/21715. Good results are achieved when a positive working precursor is given a xe2x80x9cconditioningxe2x80x9d heat treatment at a moderate temperature, for example 40-90xc2x0 C., for an extended period, for example at least 4 hours. In the method described the heat treatment is applied to a precursor or a packet of 13 precursors.
In EP-A-1074889 there is described a related method to that of WO 99/21715, in which the precursor undergoes a xe2x80x9cconditioningxe2x80x9d heat treatment step under conditions which inhibit the removal of moisture from the precursor. One method of inhibiting the removal of moisture from a precursor during the xe2x80x9cconditioningxe2x80x9d heat treatment is to wrap the precursor in a water-impermeable sheet material; another is to carry out the heat treatment in a non-drying environment, for example a humidity controlled oven.
In the method of EP-A-1074889 the xe2x80x9cconditioningxe2x80x9d heat treatment step is similar to that described in WO 99/21715, in that it preferably employs an elevated temperature for an extended period; for example 40-90xc2x0 C. for at least 4 hours. The conditioning heat treatment may be carried out on a stack of precursors.
Thus WO 99/21715 emphasizes the importance of the conditioning heat treatment step. EP-A-1074889 does likewise, and additionally emphasizes the importance of moisture during the conditioning. Neither focuses on the cooling of the heat treated precursors.
EP-A-1074386 describes a process in which a printing form precursor undergoes a heat treatment regime which includes a conditioning heat treatment as described above, followed by the controlled slow cooling of the precursor. The controlled slow cooling should take at least 1 hour, and most preferably at least 6 hours. The cooling rate should not exceed 1xc2x0 C./min, and most preferably should not exceed 0.2xc2x0 C./min. The experiments described in EP-A-1074386 are all on individual precursors, and in practice the controlled slow cooling involves allowing them to cool in the conditioning oven, which had been switched off. Additionally, the possible application of the invention of EP-A-1074386 to precursor coils or stacks of precursors is described. Controlled slow cooling of a precursor coil or a stack of precursors is defined as cooling under conditions such that heat is lost from the coil or stack more slowly than if the same coil or stack were cooled under ambient conditions.
It is an object of the present invention to provide a heat treatment which leads to printing form precursors with good properties, in an expeditious manner.
It is a further object of the present invention to reclaim off-specification printing form precursors efficiently.
This invention is directed to a method of preparing a printing form precursor and such a precursor, the method comprising:
(a) providing an imageable coating on a substrate, wherein the coating comprises a positive working polymeric composition;
(b) subjecting the precursor to a heat treatment at an elevated temperature; and
(c) accelerating the cooling of the precursor.
This invention is also directed to a method of improving the imaging characteristics of a thermally imageable lithographic printing form precursor having degraded imaging characteristics, the method comprising:
(a) heating the thermally imageable precursor to an elevated temperature in the range of 45-110xc2x0 C.; and
(b) accelerating the cooling of the heated precursor to 30xc2x0 C. or below.
This invention is also directed to a method of treating a printing form precursor, the method comprising:
(a) providing a precursor comprising a heat imageable coating on a substrate, the coating comprising a positive working polymeric composition which comprises a polymer having hydroxyl groups, an insolubilizer which acts to inhibit the dissolution of the coating in a developer prior to heat imaging but not after heat imaging, and a radiation absorbing compound able to absorb electromagnetic radiation in the range 600 to 1400 nm and covert the radiation to heat;
(b) heating the precursor, in the form of an individual precursor or a precursor web or in a packet, is subjected to an elevated temperature for a period to exceeding 5 hours; and
(c) accelerating cooling of the precursor to bring the precursor temperature to 30xc2x0 C. or below in less than 1 hour.
This invention is also directed to a method of treating a printing form precursor included in a stack, the method comprising:
(a) providing a stack comprising a precursor having a heat imageable coating on a substrate, the coating comprising a positive working polymeric composition which comprises a polymer having hydroxyl groups, an insolubilizer which acts to inhibit the dissolution of the coating in a developer prior to heat imaging but not after heat imaging, and a radiation absorbing compound able to absorb electromagnetic radiation in the range 600 to 1400 nm and convert the radiation to heat;
(b) subjecting the stack to an elevated temperature for a period not exceeding 8 hours; and
(c) accelerating the cooling of the stack to bring the stack temperature to 30xc2x0 C. or below over a period not exceeding 8 hours.
This invention is also directed to a method of producing a lithographic printing form bearing a pattern in a coating thereon and such a printing form, the method comprising:
(I) preparing a precursor by a method comprising:
(a) providing an imeagable coating on a substrate, wherein the coating comprises a positive working polymeric composition,
(b) subjecting the precursor to a heat treatment at an elevated temperature, and
(c) accelerating the cooling of the precursor;
(II) imagewise exposing the coating of the precursor; and
(III) contacting the exposed coating with an aqueous developer thereby removing imagewise exposed regions of the coating.
Other aspects of this invention will be apparent from the detailed description set forth below.